Non-imaging, computer assisted navigation system for hip replacement surgery

ABSTRACT

The invention includes: a locating system; a computer, interfaced to the locating system and interpreting the positions of tracked objects in a generic computer model of a patient&#39;s hip geometry; a software module, executable on the computer, which defines the patient&#39;s pelvic plane without reference to previously obtained radiological data, by locating at least three pelvic landmarks; and a pelvic tracking marker, fixable to the pelvic bone and trackable by the locating system, to track in real time the orientation of the defined pelvic plane. Preferably, the system also includes a femoral tracking marker, securely attachable to a femur of the patient by a non-penetrating ligature and trackable by the locating system to detect changes in leg length and femoral offset.

[0001] This application is a divisional of U.S. application Ser. No.10/075,796 filed on Feb. 13, 2002, and claims priority of thatapplication.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to computer assisted surgery generally andmore specifically to computer assisted total hip replacement (THR) orhip arthroplasty operations.

[0004] 2. Description of the Related Art

[0005] Total hip replacement or arthroplasty operations have becomeincreasingly common in the United States, with more than 300,000 suchoperations occurring annually. Many of the procedures will eventuallyrequire revision, due to one of any number of problems. Problems canarise with the implant, which can wear, degrade or even fracture. Inother cases, dislocation of the replaced hip can occur, causing extremepain (not to mention inconvenience and expense). The incidence ofdislocation has remained at approximately 2-6 percent, in spite ofimprovements to technique and materials.

[0006] It is known that the incidence of post-surgical dislocation isrelated to the orientation of the hip replacement components,particularly to the angular orientation of the acetabular shellcomponent in relation to the bony anatomy. See Lewinnek et al.,“Dislocation after total hip-replacement Arthroplasties,” Journal ofBone and Joint Surgery, Vol. 60A, No. 2, PP. 217-220 (1978). The headand neck geometry of the implant is also thought to be a factor.

[0007] In spite of the published research, the typical surgeon has notadopted any sophisticated method of navigating hip replacement surgery,in spite of the availability of several techniques. The most prevalentmethod is to rely on an acetabular impactor tool with a handle placed atan angle predetermined so that if the handle is maintained at a level,horizontal orientation, the acetabular shell will be at a desired angle.This method fails to consider the considerable movement and variation inthe patient's pelvic position during surgery; at worst it aligns theshell with the operating table (not necessarily the pelvis). Moretechnological methods have been developed, including the sophisticatedmethod described in U.S. Pat. No. 6,205,411 (and related applications)to DiGioia et al. (2001). The method of DiGioia is an advance over theprior methods (which he summarizes authoritatively in his “Background”section).

[0008] DiGioia's method begins with extensive pre-operative imaging,including relatively expensive CT scanning. The pre-operative imagery isthen input into a digital computer model, which performs extensive,three-dimensional modeling including range of motion simulations of thepatient's anatomy in relation to a specific computer model of aparticular implant. Next, in an intra-operative phase, the pre-operativemodels are registered with intra-operative optical tracking data: a verylarge number of points are sampled on the pelvis and femur, and thecomputer fits the data to the pre-operative model. Finally, the implantis positioned to align as c_ose_y as possible with the optimizedcomputer model.

[0009] The method of DiGioia et al. is complex and requiressophisticated digital and radiological techniques. A need still existsfor a simpler method of surgical navigation which will facilitate properhip geometry with a minimum of pre-operative imagery and expense. It isfrequently found that physicians are loath to adopt any methods, andparticularly any computerized methods, which are unduly complex,expensive or time consuming. In this they may be forgiven, in light ofthe increasing economic constraints which burden the modern practice ofmedicine.

[0010] Thus, a need persists for an intra-operative computer assistedhip navigation system which is easily learned, rapidly executed,economically practical, and independent from expensive or exoticpre-operative radiological imagery.

SUMMARY OF THE INVENTION

[0011] In view of the above problems, the present invention includes amethod and system for intra-operative navigation of a hip replacementoperation which permits a surgeon to intra-operatively assess theorientation of an acetabular shell implant and/or the orientation of afemoral stem implant, without recourse to pre-operative imagery orcomputerized simulations.

[0012] According to one aspect, the invention is a computer assisted,non-radiological method of intra-operatively measuring and assessingrelative geometric relationships among skeletal features of a hip joint,suitable for surgical navigation of a hip arthroplasty operation. Themethod includes the steps: defining a pelvic plane from at least threerecognizable anatomic features of the pelvis; tracking with an opticaltracking system the orientation of an acetabular implant, to obtainacetabular implant orientation data; and adjusting the acetabularimplant into a desired orientation with respect to the defined pelvicplane, without reference to previously obtained radiological data, byrelating the acetabular implant orientation data to the defined pelvicplane.

[0013] According to another aspect, the invention includes a device fortracking the upper femur, suitable for use during a hip arthroplastyoperation. The device includes: a rigid collar which can engage thegreater trochanter; an optical tracking target, mounted on the rigidcollar; and a ligature, attached to the rigid collar and capable ofbeing wrapped around the upper femur and tensioned to urge the collaragainst the greater trochanter, for attaching the rigid collar to thefemur.

[0014] Still another aspect of the invention is a method of attaching atracking marker to the upper femur, suitable for tracking the upperfemur during hip arthroplasty or a similar operation, including thesteps: positioning a rigid collar in contact with the greater trochanterof the femur, the collar adapted for mounting thereon a tracking marker;attaching a ligature or opposable clamp to the collar; wrapping theligature around the upper femur; and tightening the ligature or clamparound the femur to pull the collar tightly against the greatertrochanter.

[0015] Another aspect of the invention is a method of determiningchanges between (1) pre-operative femoral position and (2)post-operative implant geometry, suitable for use during a hiparthroplasty operation, including the steps of: Maneuvering the femurinto a reference position; measuring, with a non-radiological opticaltracking device, pre-replacement femoral parameters; after implanting aprosthetic, returning the femur to the reference position; againmeasuring, with a non-radiological optical tracking device,post-replacement femoral parameters; and comparing the pre-replacementand the post-replacement parameters in a computer model.

[0016] The system of the invention includes: a locating system; acomputer, interfaced to the locating system and interpreting thepositions of tracked objects in a generic computer model of a patient'ship geometry; a software module, executable on the computer, whichdefines the patient's pelvic plane without reference to previouslyobtained radiological data, by locating at least three pelvic landmarks;and a pelvic tracking marker, fixable to the pelvic bone and trackableby the locating system, to track in real time the orientation of thedefined pelvic plane.

[0017] Preferably, the system also includes a femoral tracking marker,securely attachable to a femur of the patient by a non-penetratingligature and trackable by the locating system to detect changes in leglength and femoral offset.

[0018] Preferably, a system in accordance with the invention alsoincludes a method of verifying reliability of a computer assisted,optically tracked surgical navigation system. According to this method,a fixed optical tracking marker is tracked in relation to a fixedreference mark on the same bone, both before and after the surgicalprocedure. This provides a check (“tracker check”) to detect any errorsdue to, for example, slippage, drift, or deformation of the apparatus.

[0019] These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a system-level block diagram of the apparatus of theinvention in a typical surgical environment;

[0021]FIG. 2 is a high-level flow diagram of the method of theinvention;

[0022]FIG. 3 is a perspective view of an optically trackable manualprobe suitable for use to input positional information in the method ofthe invention;

[0023]FIG. 4 is a perspective view of another optically trackableposition marker suitable for fixation to the pelvis for tracking theposition and orientation of the pelvis;

[0024]FIG. 5 is a perspective view of yet another optically trackablemarker adapted for fixation to the femur to track position andorientation of the femur;

[0025]FIG. 6 is a plan view of the indexing collar, also shown in FIG.5, which facilitates secure attachment of the tracking marker to thepatient's femur without penetrating devices or screws;

[0026]FIG. 7 is a perspective view of an acetabular component placementtool (“impactor”), equipped with a tracking marker in accordance withthe invention;

[0027]FIG. 8 is a flow diagram of detailed steps suitable for executingthe acquisition step of FIG. 2;

[0028]FIG. 9 is a frontal view of a pelvis, defining the pelvic plane, apelvic coordinate system, and an abduction angle;

[0029]FIG. 10 is a top view of the pelvis of FIG. 9, defining a versionangle;

[0030]FIG. 11 is a flow diagram showing detailed steps suitable forexecuting the navigation step of FIG. 2;

[0031]FIG. 12 is a frontal view of a hip joint, illustrating thegeometry and defining a relative offset and leg length, as determined inthe method of the invention;

[0032]FIG. 13 is a flow diagram showing detailed steps suitable forexecuting the verification step of FIG. 2; and

[0033]FIG. 14 is a typical printout/record produced by the method of theinvention for archiving and/or retention in the patient's medicalrecords.

DETAILED DESCRIPTION OF THE INVENTION

[0034]FIG. 1 shows a system-level block diagram of the system orapparatus 20 of the invention in a typical operating room environment. Aphysician or other professional 22 performs a hip surgery (for example,total hip replacement) on a patient 24. An optical or equivalent locatoror locating system 26 is disposed near the patient, so that theoperating field is encompassed substantially within the field of view 28of the locator 26. A suitable optical locator is available commercially,for example the “Polaris” available from Northern digital Inc., inWaterloo, Ontario, Canada. Optical trackers or markers 30 are usedduring the operation, as more fully described below in connection withFIGS. 3-7. The markers 30 allow the locator 26 to acquire the positionsand orientations of tools and anatomical reference points, as describedbelow.

[0035] The optical locator 26 is interfaced with and outputs trackingdata to a digital computer 32, which interprets the optical trackingdata as it is received. Using well known geometric relationships, thecomputer is programmed to deduce from the optical field of view theactual positions and orientations of the markers, and, by extension, thepositions and orientations of the instruments and/or anatomical featuresthat are in known relationship to the markers. For example, suitableoptical markers utilizing multiple reflective spheres are available fromTraxal, Inc. in Toronto, Ontario, Canada. Markers with active lightemitting devices such as LEDs are also available and could equivalentlybe used. Note that typical markers include three or more non-collinearcomponents; this allows the locator and computer to determine not onlythe positions but the orientation (rotation) of such a marker in space.This capability is exploited in the methods described below.

[0036] Preferably, the computer 32 is also programmed with auser-friendly interface (software) which facilitates the execution ofthe method of the invention (described below in connection with FIG. 2).The physician or other personnel can view output (for example on a videomonitor) and input instructions to the computer 32 via I/O devices 34,which suitably could include a monitor, keyboard, printer, foot pedals,and other input/output devices such as conventional “mouse” or similarpointing devices.

[0037] Preferably, the system also includes a record storage device 36such as a CD-R drive, and/or simply a printer which prints out a summaryof the operation and patient data for future reference or medicalarchiving.

[0038]FIG. 2 is a top-level flow diagram of the method of the invention,showing the steps of the method at a high level of abstraction. Specificsteps are elaborated and explicated in connection with later figures,along which particular surgical apparatus suitable for practicing themethod.

[0039] In broad terms, the method includes three major steps, allperformed intra-operatively: Acquisition of patient geometry (40),computer aided navigation of surgery (42), and computer aidedverification (44) of implant geometry.

[0040] Patient geometry is acquired (in step 40) by attaching andoptically tracking several optically trackable markers, described belowin connection with FIGS. 3-5. Note that the acquisition of patientgeometry according to this method does not utilize any radiographic orother imaging.

[0041] After acquiring the initial or “native” geometry of the patient'spelvic-femoral system, in step 42 the method uses continuous ornear-continuous, real-time optical tracking of the pelvis and femur aswell as surgical tools, including an optically trackable impactor orequivalent tool for positioning and fixing an acetabular shell implant.Computer acquired and calculated information is displayed to the surgeonin real time to facilitate placement of the acetabular shell implantwithin a desired angular range of anteversion and abduction (andpreferably also a desired range of anterior/posterior angulation or“flexion”). The femur is also tracked and computer graphic displayallows the surgeon to achieve a desired amount of femoral offset and adesired leg length (typically very nearly matching the native length andoffset on the opposite side of the body).

[0042] Finally, in step 44 the orientation and position of pelvic andfemoral tracking markers are preferably verified by optical tracking andcomputer calculation, by a method of redundant checking (“trackercheck”). This step reveals any inconsistencies, such as might occur dueto slippage or bending of instruments, or other errors. If any,significant discrepancy is revealed during verification, the surgeon hasthe option to repeat some or all of the surgical procedure beforeterminating the procedure.

[0043] Preferably, the verification step 44 also includes making andstoring permanent records of the procedure, including patient andimplant geometry, for archiving or medical reference. The record can bein machine readable, and/or human readable form. For example, a printoutis preferably generated which can be entered into a traditional medicalfiling system, together with a machine-readable record of the operation,for example on CD-R.

[0044] The more detailed procedural explanation given below in FIGS. 8through 13 makes frequent reference to certain optically trackablemarkers and tools which are specifically adapted for the invention.Visualization of the procedure thus will be greatly facilitated by firstconsidering typical trackable markers and tools.

[0045] A typical optically trackable manual probe 50 is shown in FIG. 3.This probe includes a pointable tip 52at the front end of an elongatedstem 53 of known length and shape. The rearward end of the stem 53 isfixed to a rigid body 54 of known dimensions and geometry, suitablyshaped for hand-gripping. Mounted to the body 54 is an optical trackingtarget 56 having at least three optical tracking references. Both activeand passive references, targets, and probes are available commercially,for example from Traxal, Inc. in Toronto, Ontario, Canada.

[0046] It is known that an optical tracking target such as 56, withknown dimensions and geometry, can be optically tracked for example byan optical locating system available from Northern Digital, Inc.(referenced previously). Since the dimensions and shape of the stem 53and body 54 are known, tracking of the position and orientation oftarget 56 allows ready calculation of the position of the tip 52 by wellknown geometric relationships. Thus, to enter a spatial location (suchas an anatomical landmark) into to computer 32, a physician can touchthe tip 52 to the location while simultaneous cueing the computer toinput the instant position. A foot switch is a typical and convenientmethod of cueing the input.

[0047]FIG. 4 shows a similar optically trackable pelvic marker showngenerally at 60, which includes a trackable marker 61 adapted forfixation on a bone screw 62, and suitable for intra-surgical fixation toany convenient exposed surface of the pelvis. Note that a quick releasedevice 64 is preferably provided between the target 56 and the bonescrew 62. The quick release device 64 allows the trackable marker 61 tobe quickly attached or detached as required during surgery. Detachmentof the marker 61 is convenient for the surgeon, lest he find hismovements encumbered by its presence. The quick-release device should bedesigned to provide well defined, stable, and reproducible positioningof the marker 61 with respect to the bone screw 62.

[0048]FIG. 5 shows generally at 68 a femoral tracking marker devicefixed in a typical manner on a patient's femur 69 near the greatertrochanter. It is extremely preferable that this device be fixable tothe femur in a firm and fully engaged position which does not allowslippage or rotation, but without the use of bone screws, pins or anyother bone damaging devices. Specifically, it is extremely preferablethat the marker 68 is attachable to the femur by a device which does notpenetrate the outer cortical (hard) shell of the bone. It ispermissible, in accordance with the invention, to use aggressivelytextured surfaces, which could include spikes or cleats which do notpenetrate the outer cortical shell.

[0049] Accordingly, the preferred embodiment shown employs anellipsoidal indexing collar 70 and elongated tensioning members 72 whichwrap around the femur 69. One advantageous position of the tensioningmembers 72 is shown, with independent tensioning members running aboveand below the projection of the lesser trochanter 74 to secure theplacement from axial slippage. The tensioning members 72 pull theindexing collar onto the projection of the greater trochanter 76.Because of the irregular shape of the greater trochanter 76, we havefound that the ellipsoidal indexing collar 70 tends to seek a stableposition vis-à-vis the greater trochanter 76, which position ismaintained firmly by the tensioning members 72.

[0050] A shaft 78 is fixed to the indexing collar 70 at a first end andsupports at a second end an optical tracking target 79, similar to thosepreviously described. The shaft 78 may optionally have a bend,articulation, or joint (not shown, to maintain clarity of the drawing)to allow the marker to be oriented in a better aspect for opticaltracking.

[0051] The tensioning members 72 collectively comprise a “ligature”. Itshould be understood that the term “ligature” is used in its mostgeneral sense: as something which binds, unites or connects. Thetensioning members 72 are suitably made from elastomeric fibers or cordssuch as those available from Poly 4 Medical in Goleta, Calif. Othermaterials or other flexible members such as wire, bands, mesh, spring,cords, ligaments or string could alternatively be used in a wrappingarrangement to comprise the ligature. It is desirable that thetensioning members 72 be so constructed that they are capable ofexerting sufficient force to firmly attach the marker device, but arenot so strong that a crushing force could be accidentally exerted on thefemur. To attach a typical marker device 68, the surgeon threads thetensioning members 72 through a slot (82 in FIG. 6, below), encirclesthe femur, and threads through an opposing slot (84 in FIG. 6, below).He then pulls the tensioning members to an appropriate tension andattaches a clamp 86 to hold the member in tension. Suitable clamps areavailable from Poly 4 Medical, but other means of holding the members intension could be used, such as various hooks or staples.

[0052]FIG. 6 shows a typical collar 70 in isolation. The aperture 90 ofthe collar is preferably elliptical, with major axis a and minor axis bin the range of 30 mm to 45 mm (major axis) and 20 mm to 35 mm (minoraxis). Other shapes could be used, including ellipsoidal, horseshoe, “U”or irregular shapes. A range of collars of varying sizes may beadvantageously kept available for the surgeon to accommodate variouspatients. The collar preferably has an inward facing bevel 92 atapproximately a 45 degree angle, defining an angled shoulder which ispreferably knurled or otherwise textured to grip the trochanter in apositive manner.

[0053] The femoral tracking marker 68 of the invention is extremelyadvantageous and is preferred over prior devices such as bone screwtracking devices (such as that described in U.S. Pat. No. 5,807,252 toHassfeld et al.). Such bone screw devices are commonly used in kneereplacement surgery. The upper femur, however, is less amenable to bonescrew attachment. Because of the mechanics of the hip and upper femur,the upper femur experiences very large stress and shearing forces, bothin its natural state and after implantation of an artificial hipprosthesis. In extreme cases this stress can actually cause theprosthetic stem to fracture the upper femur. Thus, it is desirable toavoid placement of any penetrating device such as a bone screw into theupper femur, as the penetration could compromise the structuralintegrity of the bone tissue. The femoral tracking device of theinvention thus permits convenient and quick attachment without fullypenetrating the outer cortical (hard) shell of the femur.

[0054] Alternatively, a clamp, fixable to the femur and/or the greatertrochanter, could be used to attach the tracking marker to the femur;but the collar and ligature described above is preferred, as it providesa convenient and secure attachment. Whatever method is used, it isextremely preferable to avoid fully penetrating the outer cortical(hard) shell of the femur.

[0055] One further trackable tool is useful. FIG. 7 shows the acetabularshell component 93 mounted on the impactor placement tool 94. Typicallythe shell component 93 is essentially a sliced spherical shell, whichmay be hemispherical or describe less than half of a sphere. Theimpactor tool 94 preferably has a shaft 95 which is fixable to the shell93, for example by a pressed fit or threaded mating device. Once fixed,the shaft is held at a known orientation with respect to the shell. Itis particularly convenient if the shaft 95 is fixed normal to the planeof the lip 96, as shown in the figure. An optically trackable marker 98is mounted to the impactor shaft 95, but offset by a secondary shaft 100(which may optionally include a quick release device). The marker 98 isheld in fixed angular relation to the impactor shaft 95, so that bylocating the orientation of marker 98, the angle of the shaft 95 iseasily also determined. This tool is employed during the “navigation”step of the method (described below). The preceding discussion of thepreferred optical tracking markers should be borne in mind whileconsidering the following detailed procedural descriptions of thepreferred method of the invention.

[0056]FIG. 8 shows in greater detail preferred steps which are includedin the acquisition step (40 in FIG. 2). Preliminary steps 140-144 areconventional. In step 140, the system is initialized: a welcome screenappears and the physician or other professional enters relevant patientand physician information. Next (step 142) the physician checks thefield of view with an optically trackable pointer such as that describedabove in connection with FIG. 3. The surgical field should be arrangedto lie substantially within the field of view 28 of the optical tracker26 (in FIG. 1) yet close enough to the optical tracker to allow a highdegree of tracking accuracy. Next the patient is prepared for surgery ina conventional manner and introduced into the field of view (step 144).

[0057] Note that no pre-operative computer modeling or high-resolutionradiological imaging (such as a CAT scan) are included in the method ofthe invention (although a physician typically will have consultedprevious X-ray images before surgery).

[0058] Next, in step 146, the physician attaches at least one pelvicmarker 60 and at least one femoral tracking device 68 (discussed abovein connection with FIGS. 4 and 5). The pelvic tracking marker 60 issuitably attached by inserting a bone screw or other fixing device intoan exposed portion of the pelvic bone. In contrast, according to theinvention the femoral tracking marker 68 is attached without penetratingbone screws, as described previously. By avoiding insertion of bonescrews into the femur, the invention prevents injury or mechanicalcompromise of the highly stressed upper femur, thereby lessening thelikelihood of post-operative complications due to femoral fracture.

[0059] Next, in pelvic definition step 148, the physician uses anoptically trackable manual probe 50 to palpate at least three, andpreferably four, easily located anatomical landmarks on the pelvis. Thisis accomplished, for each landmark, by activating a foot pedal or otherswitch while simultaneously positioning the probe in percutaneouscontact overlying a prominent anatomical landmark. When thus cued, thecomputer 32 receives apparent positional information regarding the probefrom the optical tracking system 26 and calculates from this informationa position for the corresponding landmark in a reference frame attachedto the pelvic marker 60.

[0060] The reference landmarks in the pelvic definition step 148 aresuitably chosen from: the ipsilateral anterior superior iliac spine(ipsilateral “ASIS”), a contralateral anterior superior iliac spine(contralateral “ASIS”), an ipsilateral pubic tubercle, and acontralateral pubic tubercle. Basic geometry dictates that at leastthree points are required to define a plane. However, preferably allfour of the above mentioned reference landmarks should be input into thecomputer system to better define a pelvic plane. One suitable method isto define an imaginary point at the midpoint, of the line segmentbetween the two pubic tubercles. This midpoint is then used, togetherwith the two ASIS, to define the pelvic plane. Suitably, the computercan choose a plane by a least squares minimum error fit to the fourpoints, if any asymmetry exists. A Pelvic Coordinate frame of referenceis also defined in this step, suitably with origin at the midpointbetween the ASIS. a suitable coordinate frame is more fully describedbelow in connection with FIGS. 9 and 10.

[0061] Note that the pelvic reference plane (“anterior pelvic plane”) isan imaginary plane defined by 3 points; no effort is made to curve fitto a complete, non-planar model of the pelvic bone. Indeed, no suchmodel is assumed to be available, as no pre-operative CT or MRI scan isrequired by the method.

[0062] Next, in step 150, the computer relates the pelvic referenceplane (calculated from step 148) to the reference frame of the pelvictracking marker. That is to say that the pelvic tracking marker, firmlyattached to the pelvic bone at some hitherto unknown orientation,defines a pelvic tracking marker reference frame (PTMRF, an orientationand position of the marker). The pelvic coordinate system that wascalculated in step 148 is related to the PTMRF by a rotation andtranslation, and this relationship is calculated and stored. Differentlystated: The pelvic reference frame found by palpating landmarks definesa first coordinate system; the position and orientation of the fixedpelvic tracking marker defines a second coordinate system, related tothe first by an affine transformation. The affine transformation (andpreferably the inverse transformation) are calculated by well knownmeans and stored.

[0063] Next, in optional step 152, a redundant accuracy check (“Trackercheck”) is initialized. Preferably, a redundant reference mark is placedon the pelvis at some position other than that of the fixed pelvictracking marker. Cauterization is a suitable and convenient method ofmarking the pelvis, but other methods could also be used. The physicianthen touches the redundant reference mark with an optical tracking probewhile vector thus defined is stored for future reference (duringnavigation steps, described below).

[0064] Next, (step 154) the physician (in coordination with the programexecution of computer 32) pivots the femur, typically in arcs or circlesconsistent with its natural arcs of movement. The movements of thefemoral tracking marker are tracked by the optical locating system 26and interpreted by the computer 32 to calculate the natural or “native”femoral head center (referred to as “C1”). This is suitably accomplishedby assuming that the motion of a point on the femur is constrained tolie on a partial spherical surface with its center at the native headcenter. A least squares surface fitting algorithm is suitably used tocalculate the center of the sphere (C1).

[0065] After finding the native head center, the physician disposes thefemur in a natural reference position (“Position 1”), preferably alignedwith the patient's spinal axis, while cueing the computer to initializeoffset and leg length (step 156) by storing the tracker position forfuture comparison. Specifically, The position of the femoral trackingmarker 68 is located by optical locating system 26 and the data isinterpreted by the computer 32. The position of the femoral trackingmarker 68 essentially defines a position on the femur; this position isrelated to the pelvic tracking marker and hence the PTMRF by somerelative offset and length (“leg length”) which are calculated andstored for future comparison (in navigation steps, described below).Note that the offset and leg length calculated provide an arbitraryreference for relative comparison. The measurements are not absolute,and are useful only so long as the femoral tracking marker 68 remainsfixed with respect to the femur and the pelvic tracking marker remainsfixed in relation to the pelvis. Nevertheless, the relative positioninformation suffices to permit meaningful comparison of thepre-operative with the post-operative leg length and offset.

[0066] These steps complete the initial acquisition of geometry (step 40of FIG. 1).

[0067]FIGS. 9 and 10 show the pelvis and define the pelvic plane andpelvic coordinate system which references the angle of the acetabularshell implant. Right and left pubic tubercles 160 a and 160 b are shown,as well as the midpoint 161, in relation to right and left anteriorsuperior iliac spines (right 162 a and left 162 b). All four typicallylie on or near the pelvic plane 164; we define the plane by the threepoints: Right and left ASIS 162 a and 162 b and the midpoint 161 betweenthe pubic tubercles. We define an origin O on the pelvic plane andlocated halfway between the right and left ASIS 162 a and 162 b. Fromthe origin and pelvic plane we define right-handed, orthogonal Cartesiancoordinates as shown, such that the XY plane is the pelvic plane and a Zaxis is normal to the pelvic plane intersecting at origin O. Anacetabular opening 168 is shown in pelvis 170. We can define thesignificant angles of the acetabular shell component, relative to ourpelvic coordinate system. We define the axis 172 of the shell componentas a vector normal to the plane defined by the rim of the component. Thevector 172 intersects the plane at the center of the circle of describedby the rim. With the axis 172 thus defined, we can define itsorientation by θz (abduction), θy (version) and θx (anterior/posteriorflexion). θz defines rotation about the z axis; it is shown as the angleof the projection of the vector 172 into the XY plane. Similarly, θydefines rotation about the Y axis; it is shown as the angle of theprojection of 172 into the XZ plane. The abduction angle θz isconventionally measured from the negative Y axis; the version angle,from the negative X axis (for a right leg as shown) or the X axis (for aleft leg). The third angle (“flexion”, not shown) similarly definesrotation about the X axis, and is conventionally measured from thenegative y axis. Flexion angle should preferably should also be measuredand displayed in the method of the invention.

[0068] Note that these coordinates are equivalent to the “Anatomical”reference frame defined by and Jaramaz et al. in “Computer AssistedMeasurement of Cup Placement in Total Hip Replacement,” in ClinicalOrthopaedics and Related Research, No. 354, pp. 71-81 (1998, Lippincott,Williams and Wilkins) (their FIG. 2). We have used the normal vector 172in place of the cup plane used by Jaramaz, for ease of visualization;but both define the orientation of the cup in an equivalent way. Notethat other reference frames such as the “Radiographic definition” andthe “Operative definition” are also frequently used in the literature. Adefinition in the Anatomic reference frame can be converted to eitherthe Radiographic or Operative reference frame by mathematicaltransformation (preferably performed by computer 32). Please refer tothe Jaramaz article, op. cit., for more details on the various frames ofreference.

[0069]FIG. 11 shows detailed steps of the surgical navigation step ofthe method (step 42 of FIG. 2, above). References to version andabduction can be easily visualized by reference back to FIGS. 9 and 10above.

[0070] With reference to FIG. 11 next (in conventional surgical step200) the physician will dislocate the hip and ream the acetabulum toprepare for location of an acetabular implant component (“shell”). Thesetechniques are well known in the surgical arts and are not describedhere.

[0071] Once the acetabulum is prepared for the implant, the physicianuses an optically trackable impactor tool 94 (described previously inconnection with FIG. 7) along with the tracking system, to navigateplacement of the acetabular implant shell. Specifically, the trackableimpactor tool is fixed to the implantable cup. The surgeon moves thetool through a series of angles. The tool is tracked by the opticaltracking system and the angles displayed via the computer 32 on thedisplay device 34, thus providing feedback as to the abduction angle,version angle and flexion angle of the implant cup relative to theanterior pelvic plane. When the computer indicates that the desiredangles have been attained, the surgeon uses impact to firmly place theacetabular cup component. A specific angle is not mandated by theinvention, but rather the choice of the angle is left to the physician.Fixing screws of various types can also be used to fix the shell, as isknown in the medical arts.

[0072] In the navigation step 202 the optical tracker 26 allows thecomputer 32 to calculate an orientation of the long axis 95 of theimpactor shaft, which is fixed in known relation to the acetabular shellcomponent. This orientation is then compared with the real-timeorientation of the pelvic reference plane, as determined in real time bytracking the fixed pelvic marker (implanted in step 146 above) andthereafter applying the inverse transformation (previously determinedfrom step 150 above). The physician manipulates the tool to align with adesired abduction angle and version angle (determined as discussed belowin connection with FIG. 9). Note that the method does not require thepatient to remain immobile between defining the pelvic plane (step 148)and navigation (step 202), because any motion of the pelvis is trackedby the fixed pelvic tracking marker 60. Thus, in the computer graphicmodel the computer moves the pelvic reference plane in concert with anypelvic displacement or rotation, in real time.

[0073] To properly align the shell, the physician moves the impactortentatively while observing the display (34 in FIG. 1) for feedback. Theoptical tracking system 26 and computer 32 track in real time theorientation of the impactor tool 212 and display the orientation, alongwith some target or reference pattern (for example, a cross-hairs targetor two protractor displays, one for version and the other for abductionangle). It has been found that 45 degrees of abduction (θz) and 20degrees of version (θy) will typically yield an acceptable result(minimize the number of post-surgical dislocations). A range of 40degrees±10 (abduction) and 15 degrees±10 (version) is acceptable,measured in the radiographic frame. The precise angle and range isentrusted to the discretion of the physician, based on his experienceand available literature. See DiGioia et al., “Image Guided NavigationSystem to Measure Intraoperatively Acetabular Implant Alignment,”Clinical Orthopaedics and Related Research, No. 355, pp 8-22 (1998Lippincott, Williams and Wilkins); Lewinnek, et al., “Dislocations afterTotal Hip Replacement Arthroplasties,” Journal of Bone and JointSurgery, Vol 60A, No. 2, (March 1978). Once the proper orientation hasbeen established, the shell component is set by impaction and/or screws,according to the implant system.

[0074] The orientation of the implant shell 93 is preferably nextverified (step 204) by touching at least three points on the rim of theacetabular implant shell 93 with the tip 52 of probe 50 and inputtingthe three positions via the locating system 26. The three or more pointsare used by the computer to define the plane of the shell opening, whichis normal to a vector 172. The orientation angles of the vector 172 (orequivalently, that of the plane of the shell opening) is then displayedto the physician and preferably recorded for future reference.Preferably, all of angles θx, θy and θz are displayed and recorded.

[0075] Typically a polyethylene liner is then fixed to the shell, as isknown in the orthopaedic arts. Before fixing the liner, however, theliner position can be adjusted. Today's modular liners typically allowfor independent adjustment of the position and orientation of themodular liner within the shell. Preferably, in step 206 the physiciancan capture the contour of the liner by touching at least three (andpreferably more) points on the shell rim with the trackable manualprobe. The optical locator and computer capture contour of the liner andpreferably calculate the opening angle, orientation, of the liner, aswell as the contour of the liner rim or lip. Typical liners are nothemispherical, but may have a complex shape including, for example, anextended lip or a complex chamfer. The calculated angles are thendisplayed to the user. The liner is then typically fixed in the shell. Astill further check of proper liner placement can be optionallyperformed by again touching at least three points on the liner rim (step206) to verify the position after fixation.

[0076] These steps complete the placement of the acetabular shellcomponent (and liner). Next, the physician proceeds with the femoralcomponent (stem and head) of the hip replacement.

[0077] Next the physician will implant (step 208) a trial femoral stemand head by conventional surgical methods not described here, and thehip is reduced. An illustrative surgery with narration is published byKinamed, Inc. under the title “Total Hip Arthroplasty.” The physicianthen pivots the femur (step 210) about a series of mechanically naturalarcs while the computer and optical tracking system track the femoraltracking marker (in relation to the pelvic tracking marker). The set ofpoints thus acquired define a portion of a sphere with center at the newhead center. The computer calculates and stores the change in headcenter (from that calculated in step 154, above).

[0078] The physician will then return the femur to a natural referenceposition, “position 1” established in step 154 above. This position isachieved by moving the femur to a position in which the orientation ofthe femoral tracking marker approximately matches the orientationpreviously initialized (in step 156). The tracking system once againinputs the position of the femoral marker (step 212) and calculates thechange in leg length and offset as compared with those initialized instep 156, above. Note that the absolute leg length and offset are at notime required. For successful surgical navigation it suffices to trackand calculate the difference between the initial femoral trackerposition (while the leg is in position 1) and the later tracker position(step 212, while leg is in position 1 again).

[0079] In general, steps 210 and 212 will calculate some change frominitial head center, offset and leg length. This change may beinsignificant, but if in the judgment of the surgeon it is significant(and undesirable) he may either (1) change the prosthetic head/neck tobetter approach a desired geometry, or (2) drive the implant stem deeperinto the femur. The steps 210 and/or 212 would then be repeated until andesirable geometry is obtained. In many cases, the desired geometry maybe a change from the pre-operative leg length and offset. The choice iswithin the discretion and control of the surgeon. This completes the“navigation” step (step 42 of FIG. 2).

[0080]FIG. 12 shows the geometry involved in the method of determiningthe change in both leg length and offset using optical tracking. Point230 represents the position vector of an arbitrary point on the femoraltracker as tracked and acquired during acquisition step 154. Point 232represents the position vector of the corresponding point of the femoraltracker, as sampled after the implantation in step 212. Small vector 234represents the vector subtraction of the vector 232 from the vector 230.The vector subtraction is readily calculated, and it can be decomposedinto components (projections) in any desired plane, by conventionalvector geometry. The projection into the pelvic plane is convenient, butthat into the central coronal plane could also be used.

[0081]FIG. 13 shows details of the optional but highly preferredverification step (44 in FIG. 2). First, in synchrony with programexecution, the physician uses the probe tip (52 FIG. 3) to touch thereference mark (cauterization or equivalent) which he previously made instep 152. The optical locator and computer calculate (step 244) therelationship between the reference mark and the fixed pelvic trackingmarker (60 in FIG. 4) For example, the computer might calculate theposition of the reference mark in the pelvic marker's reference frame(PTMRF). This position is compared with the previously calculatedposition of the marker in the same reference frame (from step 152) andthe result is output for the physician's information. Based on theresult, he/she may then either choose (step 246) to revise the procedure(via return path 248, because the check shows that something moved) orend the surgery (if the check shows insignificant movement). Thisprocedure provides a redundant “tracker check” feature which reassuresthe physician that the tracking accuracy has not been compromised due tounintentional tracking marker movement.

[0082] Preferably, a similar tracker check procedure should be performedto check the fixation of the femoral tracking marker: duringinitialization the physician may make a reference mark on the femur,then after the implantation he can touch the mark and check for slippageby finding the coordinates of the reference mark in the reference frameof the femoral tracking marker 68.

[0083] Finally, it is highly desirable that the system records apermanent record of the procedure, or at least a summary suitable forinclusion into the patient's file. FIG. 14 shows a typical screencapture or printout which includes acetabular shell version andabduction (as measured by the impactor tool), shell angles, linerangles, head center change, leg length change, and leg offset change. Itis also convenient to provide a machine readable record of the surgery,on a medium such as CD-R or its equivalent.

[0084] While several illustrative embodiments of the invention have beenshown and described, numerous variations and alternate embodiments willoccur to those skilled in the art. In some operations the acetabularimplant might not be required, but the femoral navigation methods andapparatus of the invention are still applicable. The procedure may berepeated on both sides of the body in a bi-lateral THR operation.Different elastomeric straps, fibers, cords, mesh, wire, adhesives orligatures could be employed in connection with the femoral trackingmarker device. The fixed pelvic marker could also be fixed by alternatemethods such as clamps, pins or even adhesives. The method can beadapted to various body geometries and sizes, and indeed could even beadapted, with small modifications, for veterinary medicine. Trackingmeans other than but equivalent to optical could be substituted, such asradio, microwave, magnetic, sonic or ultrasonic tracking systems,provided that the system be not so clumsy or bulky as to interfere withthe surgical manipulations required. The geometries of the various toolsand markers can be varied or modified to accommodate different trackingapproaches. Active or passive optical targets can be used on thetracking markers. Such variations and alternate embodiments arecontemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

We claim:
 1. A computer assisted, non-radiological method ofintra-operatively measuring and assessing relative geometricrelationships among skeletal features of a hip joint, suitable forsurgical navigation of a hip arthroplasty operation, comprising thesteps of: defining a pelvic plane from at least three recognizableanatomic features of the pelvis; tracking with a locating system theorientation of an acetabular implant, to obtain acetabular implantorientation data; and adjusting said acetabular implant into a desiredorientation with respect to said defined pelvic plane, without referenceto previously obtained radiological data, by relating said acetabularimplant orientation data to said defined pelvic plane.
 2. The method ofclaim 1, wherein said step of defining a pelvic plane comprisestouching, with a trackable probe, superficial points corresponding tosaid anatomic pelvic features, and tracking said probe with saidlocating system.
 3. The method of claim 2 wherein said anatomical pelvicfeatures comprise at least three of an ipsilateral anterior superioriliac spine, a contralateral anterior superior iliac spine, anipsilateral pubic tubercle, and a contralateral pubic tubercle.
 4. Themethod of claim 2 further comprising the steps of: fixing a trackablemarker on the pelvic bone, defining a pelvic marker reference system,associated with said pelvic marker; determining a relationship betweensaid pelvic marker reference system and said pelvic plane, tracking saidpelvic plane by (a) tracking said reference marker with said locatingsystem, and (b) applying a transformation to compensate for thedetermined relationship between said reference marker and said pelvicplane.
 5. A method of determining changes between pre-operative andpost-operative relationships between a femur and a pelvis, suitable foruse during a hip arthroplasty operation, comprising the steps of:maneuvering the femur into a reference position; measuring, with anon-radiological locating system, pre-replacement femoral parameters inrelation to the pelvis; after implanting a prosthetic, returning thefemur to the reference position; again measuring, with anon-radiological locating system, post-replacement femoral parameters inrelation to the pelvis; and comparing said pre-replacement and saidpost-replacement parameters in a computer model.
 6. The method of claim5, wherein said acts of measuring and again measuring the femoralparameters are performed by fixing an optically trackable marker to thefemur without penetrating through the outer cortical shell of the femur.7. The method of claim 5, including the further step of: beforemeasuring, attaching an optical tracking marker to the femur by:positioning a collar over the trochanter, said collar bearing an opticaltracking target; and attaching said collar to the femur by a ligatureabout the femur, said ligature arranged to pull the collar firmlyagainst the trochanter.
 8. A system for measuring and assessing theskeletal geometry of a hip joint during surgery, suitable for surgicalnavigation of a hip arthroplasty operation, comprising: an locatingsystem which determines positions and orientations of trackable markers;a computer, interfaced to said locating system to receive tracking data,and calculating from said tracking data the positions of tracked objectsin relation to a generic computer model of a patient's hip geometry; asoftware module, executable on said computer, which defines thepatient's pelvic plane without reference to previously obtainedradiological data, by locating at least three pelvic landmarks; and is apelvic tracking marker, fixable to the pelvic bone and tracked by saidlocating system, to track in real time the orientation of said pelvicplane.
 9. The system of claim 8, further comprising: a femoral trackingmarker, securely attachable to a femur of the patient by anon-penetrating ligature and trackable by said locating system to detectchanges in leg length and femoral offset.
 10. The system of claim 9,further comprising a trackable acetabular navigation tool, capable offixation to an acetabular shell implant; and wherein said softwaremodule calculates the relationship between said navigation tool and areal time orientation of said pelvic plane, and displays saidrelationship, to facilitate establishing proper geometry of said shellimplant during surgery.
 11. The system of claim 9, further comprising antrackable, manual probe for acquiring the positions of said pelviclandmarks, and wherein said software module defines said pelvic planefrom at least three and not more than four pelvic landmarks.